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Bacteriophages (phages for short) are viruses that infect only bacteria. The most widely studied bacteriophages are lambda (λ) and the “T even” phages, T2 and T4. The viruses contain DNA as their genetic material. The basic structure consists of the head and tail regions (see Figure 7.1). The head contains the protein coat (called a capsid) and the DNA. The tail is made up of a tube called a sheath, and, in the T even phages, several long tail fibers connected to the base of the sheath. The tail region attaches to a bacterial cell, and the DNA is injected into the cell through the sheath.

Once the DNA is inside the cell, two pathways are possible (λcan follow either pathway; the T even phages only follow the lytic cycle):
The lytic cycle: The phage DNA instructs the cell to produce more viral particles. The cell lyses, or breaks, resulting in cell death. New bacteriophages are released and can infect other cells.
The lysogenic cycle: Once infected, the virus enters a latent period, and the host cell is neither damaged nor destroyed. The phage DNA incorporates into the host cell chromosome and is replicated along with the host DNA. At this stage, the virus is technically called a prophage. The viral DNA, therefore, is passed on during cell division. Under appropriate conditions, the phage DNA will excise itself from the chromosome and enter the lytic cycle, thus destroying the host cell.

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